The mechanical properties of concrete mixed with composite coarse-aggregate were studied by the orthogonal test method, in which the four factors such as the cement content, the percent of cement replaced by fly ash, the percent of crushed limestone with grain size of 16-25 mm replaced by lightweight aggregate with the same grain size and the water to binder (cement + fly ash) ratio were considered. The results show that the water to binder ratio is the most effective factor influencing all of the mechanical properties, the mechanical properties except the splitting tensile strength are less influenced by the cement content, the elastic modulus and axial compressive strength as well as flexural tensile strength is largely affected by the replacement of cement by fly ash. The effect of the replacement of crushed limestone by lightweight aggregate is much more obvious on the cubic compressive strength than on the axial compressive strength, and larger on the flexural tensile strength than the splitting tensile strength.